This invention relates to an optical pumping module for a laser comprising a cylindrical reflector with a polygonal base.
A laser based on the invention can be used for industrial applications, particularly for cutting, welding, surface hardening materials and for marking of objects.
It may also be used for applications in the medical domain.
It is known that a laser essentially comprises an amplifying medium and two mirrors forming a resonant cavity, the amplifying medium being placed between these two mirrors.
The energy necessary for operation of a laser may be supplied electrically, chemically or optically to the amplifying medium.
In this invention, we are interested in the third manner, in other words what is called optical pumping of the amplifying medium, and more precisely transverse optical pumping of this amplifying medium.
In order to reduce effects that limit laser performances, it is important to distribute pumping light in the amplifying medium as uniformly as possible.
If nothing is done, the distribution of this pumping light is usually non-homogenous and often has a maximum at the source of this light.
The invention relates to a means of making this distribution homogenous and its advantages over existing techniques are the simplicity of implementation and construction, and therefore lower cost.
It is known that the amplifying medium of a laser absorbs all or some of the pumping power and that a given quantity is re-emitted in the form of a stimulated emission, the rest of the absorbed power being transformed into heat. This stimulated emission is called the xe2x80x9claser effectxe2x80x9d.
Absorption of the pumping power follows an exponential law (Beer-Lambert""s law) that is translated by a higher power being absorbed on the part(s) of the amplifying medium close to the pumping source. This creates pumping non-homogeneities; the power absorbed is not the same at all points in the amplifying medium.
Variations in pumping at different points in the amplifying medium also create local variations in the refraction index that result in deformation of the phase of the emitted laser beam.
The final consequence of this pumping non-homogeneities is a limitation to the quality of this laser beam. In particular, the deformation of the phase limits the extracted power and increases the divergence of the laser beam.
One known method of overcoming these disadvantages is to choose an amplifying medium that is a relatively poor absorber at the wavelength of the pumping radiation and a reflector capable of redirecting the pumping radiation that was not absorbed in the first pass, to the amplifying medium. After several passes through the amplifying medium, the pumping radiation is eventually fully absorbed.
Usually, known reflectors have a curved surface, with a length approximately the same length as the amplifying medium in order to reconcentrate unabsorbed power towards the amplifying medium.
Reflectors with curved surface are described in the following documents:
T. Brand, I. Schmidt, xe2x80x9cDesign and performance of a compact 600 W cw Nd:YAG rod laser system pumped by microchannel-cooled stacked diode laser arraysxe2x80x9d, CMA2, CLEO Europe 96
S. Fujikawa, T. Kojima and K. Yasui, xe2x80x9cHigh-power high efficient diode-side-pumped Nd:YAG laserxe2x80x9d, published by C. R. Pollock and W. Bodsenberg, OSA TOPS vol. 10, pp. 296-299.
K. Du et al., xe2x80x9cNeodymium:YAG 30-W cw laser side pumped by three diode laser barsxe2x80x9d, Appl. Opt., Vol. 37, No. 12, Apr. 20, 1998, pp. 2361-2364.
A reflector is usually machined from a metallic part that is then polished (to obtain an optical quality polish) and coated with a reflecting layer made of gold, silver or aluminium. The quality of the reflector is better when the polishing quality is better.
Curved concave surfaces with low radii of curvature (as for the surfaces considered in this case) are difficult to polish correctly. Similarly, it is difficult to apply the reflecting layer uniformly.
Reflectors may also be machined from diffusing materials such as some ceramics or some PTFE (Teflon [registered trademark]).
The disadvantage of these materials is their bad thermal conductivity that makes dissipation of heat generated by residual absorption of these materials more difficult. Since they are also porous, their use sometimes requires an additional treatment (enamelling) when they come into direct contact with a cooling fluid.
Another technique for making reflectors consists of compressing a diffusing powder (for example MgO powder or BaSO4 powder) in the space between two pieces of quartz.
This type of reflector may be long, difficult and expensive to make.
The amplifying medium of a laser may usually be considered as a convergent lens with regard to the pumping light source. This is the case particularly for the very frequent configuration of a solid amplifying medium forming a cylindrical bar with a circular base.
This is illustrated by FIG. 1 that shows a diagrammatic cross-sectional view of such an amplifying medium perpendicular to the X-axis of the bar.
If a reflecting plane 2 and the pumping light source 4 are placed facing each other on opposite sides of the amplifying medium 6, the pumping beam 8 will be refocused on or close to this amplifying medium.
This has the disadvantage that it makes the pumping non-homogeneous in the amplifying medium.
This type of disadvantage exists for the reflector with a polygonal cross-section described in the document by:
Y. Hirano et al., xe2x80x9cHigh-average-power conductive-cooled diode-pumped Nd:YLF Laserxe2x80x9d, Conference on Lasers and Electro-optics, vol. 6, 1998, OSA Technical Digest Series (OSA Washington D.C., 1998), pp. 103-104 in which the optical pumping light sources (laser diodes) are facing the reflector planes.
The purpose of this invention is to overcome this disadvantage of non-homogeneity of the optical pumping.
Its purpose is a laser optical pumping module, this module comprising an amplifying medium forming a cylindrical bar with an approximately circular base, at least one light source provided for transverse optical pumping of this medium, and a reflector that surrounds this medium and that is designed to send the light from the source along several passes towards the amplifying medium in order to homogenize pumping of the medium, this reflector forming a cylinder, the base of which is an approximately regular polygon to create several image point-sources by a kaleidoscope effect, the edges of this cylinder being parallel to the axis of the amplifying medium, this module being characterized in that the light source is facing one edge of this cylinder opposite this edge with respect to the amplifying medium, the distance between this medium and this source being chosen to optimise the homogenisation effect of optical pumping.
According to a preferred embodiment of the module according to the invention, the amplifying medium and the reflector are approximately coaxial. This thus facilitates manufacturing of this module.
Preferably, the length of the reflector is approximately the same as the length of the amplifying medium. This means that the entire length of this amplifying medium can be used for optical pumping.
According to a first particular embodiment of the module according to the invention, the number of faces on the reflector is odd and the light source is at approximately the same level as and in the middle of a face of this reflector.
According to a second particular embodiment, the number of faces on the reflector is even and the light source is approximately on one edge of this reflector.
The module according to the invention may also comprise several blocks, each block comprising at least one plane face capable of reflecting the light from the source, each face of the reflector being formed by at least one of the plane faces of the blocks.
According to a particular embodiment of the module, the light source is placed in an interval formed between two of the blocks such that light emerges from the space thus formed between the plane faces of these two blocks.
According to a first particular embodiment of the invention, the light source is a light emitter.
This light emitter may comprise a laser diode or a strip of laser diodes or a row of strips of laser diodes, or a stack of strips of laser diodes, or a combination of these two geometries, this or these strips being parallel to the axis of the cylinder formed by the reflector. When the light emitter comprises several superposed diode strips, the space between two strips preferably remains small.
In this case in which the emitter comprises a laser diode or one or more strips of laser diodes, when the light source is placed in the interval formed between the two blocks, the two blocks may be electrically conducting, the laser diode or the strip(s) of laser diodes then being electrically powered through these two blocks.
According to a second particular embodiment of the module, each light source is a light propagation means, the first end of which is intended to illuminate the amplifying medium and the second end of which is intended to receive the light from a light emitter.